The concept of spontaneous symmetry breaking is also closely related to the one
of collective variables. In molecular systems, for example, the
separation of all degrees of freedom into slow, collective ones associated with
the coordinates \(R_i\)  of the ions and the fast, intrinsic degrees of freedom
related to the electrons is straightforward. In nuclear systems, this
distinction is less clear but still applies. As recognized a long time ago,
the low-lying spectrum of nuclei contains sequences of excited states that
suggest strong collective behavior of the nucleus as whole, akin to collective
vibrations and rotations [1]}, [2]}. The order parameters of each
of the symmetry groups of the Hamiltonian can in fact provide an initial set of
collective variables. For example, the multipole moments of the one-body
density
\(\rho _{\lambda \mu }= \mathinner {\langle {\hat{Q}_{\lambda \mu }\rho }\rangle }= \int d^{3}\mathbf {r}\, Q_{\lambda \mu }(\mathbf {r})\rho (\mathbf {r})\) 
